Replacing heater matrix caused coolant temp to drop when heating turned on

[Damask_Rose]

Replaced the heater matrix in my mark 7 1.4tsi yesterday and heating is now piping hot.

But now when I turn on the heating, the coolant temp drops, especially at idle.

So I graphed the temp using OBD11 on a 20 minute long drive. The drops below 90 are where I turn the heater on and the drop to the plateau at 60 is when I parked up with the heater on.

https://i.postimg.cc/tRNGgnHd/coolant-temp-chart-golfmk7-petrol-9-Feb2024.png

The car gets up to temp fairly quickly. It will stay at 90 unless I turn the heating on.

I have no loss of coolant and no leaks visible anywhere.

I have scanned the car and it has no error codes.

I had the thermostat replaced by a highly reputable garage in 2022 and the water pump replaced by the same garage about 6 months ago.

The problem only emerged after I replaced the heater matrix.

The heat coming out the vents is still very warm even when the temperature drops.

I believe that the temperature should stay at 90 once it gets there.

So I am a bit puzzled and stuck.

I would be extremely grateful for any help or advice.

 

[YamR1rider]

What does the actual gauge do in the car - does it just sit in its usual place?

Some fluctuation in coolant temp is to be expected when you turn your heater on as coolant is flowing through all the extra piping and the matrix. If the gauge in the car is stable (it usually takes something fairly substantial to move it) and it isn't throwing throwing codes, I don't think I'd be overly concerned.

 

[Damask_Rose]

What does the actual gauge do in the car - does it just sit in its usual place?

Some fluctuation in coolant temp is to be expected when you turn your heater on as coolant is flowing through all the extra piping and the matrix. If the gauge in the car is stable (it usually takes something fairly substantial to move it) and it isn't throwing throwing codes, I don't think I'd be overly concerned.

Actual gauge does drop but only when put on very high heat combined with high blower fan setting.

But I checked my wife's Audi A4 1.4 TFSI which behaves similarly under the same conditions.

 

[YamR1rider]

Yeah, wouldn't worry about it then. Back in the day, I had an old tuned up Mini1275GT which if given some stick was prone to overheating....cranking the heater to full blast (even in the summer) really helped!

 

[Marcellus]

Yeah, wouldn't worry about it then. Back in the day, I had an old tuned up Mini1275GT which if given some stick was prone to overheating....cranking the heater to full blast (even in the summer) really helped!

It's surprising how certain odd tactics can save the day! Cranking the heater on an overheating Mini1275GT may seem unusual, but sometimes the unconventional remedies work best. .

 

[GTIscream]

The heating coil is just basically a radiator, right?
i remember having to do this same thing on an overheat in a distant life.